The present invention pertains to wet-mechanical separation of solids of different densities from mixtures of solid materials. The mixtures of solid materials may include, for example, home or industrial refuse mixtures. Such mixtures may consist of plastics contaminated with mineral heavy materials such as sands, or with other specifically heavy contaminants, for example metals. A separating liquid, whose density preferably lies between the densities of the solid materials to be separated, is added to the mixture of solid materials. Separation occurs in the centrifugal field of the rotating suspension formed from the solid materials and the separating liquid.
The wet-mechanical separation of solid materials according to density under the influence of centrifugal forces is already known.
In German document number DE-PS 29 00 666, a method for the separation of plastic waste is specified in which the separation ensues in the rotating suspension stream of a hydrocyclone. A disadvantage of the exclusive separation according to this method is the unsatisfactory separation effect, which leads to loss of solid material. Also, the required separation of the separating liquid from the solid matter fraction suspensions obtained must be implemented downstream.
These disadvantages are largely avoided in the separating centrifuge specified in German document number DE-PS 42 08 104, in which the separation ensues in the generated centrifugal field of a rotating container.
In this known, proven method for the separation of plastic mixtures in a separating centrifuge, the following problems can arise in some applications. If the material to be processed contains very large portions of fine-grained mineral contaminants (mineral heavy materials), an increased wear takes place on components of the separating centrifuge. In some applications there is a first stage in which heavy plastics have to be separated from light plastics and a second stage in which the heavy plastic (now as floating material) has to be separated from still heavier mineral contaminants (e.g. metallic strands in cable scrap). In such applications at separation densities of more than about 1.4 g/cm.sup.3, the second stage is sometimes no longer economical, due among other things to the high consumption of salt.
Known methods for separating plastic mixtures include several settling/floating stages or hydrocyclone stages which are successively connected. Such methods already known for separating plastic mixtures always require more than two separating stages because the separation effect of the individual layers is significantly lower than that of the separating centrifuge. This leads to higher consumption of water and energy, whereby the losses of valuable plastic increase with each stage.
Thus, there are needs to improve methods and apparatuses for the wet-mechanical separation of solids of different densities. The present invention solves these and other needs.